linux-kernel - Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages

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Message-Id: <548e33cb-e737-bb39-91a3-f66ee9211262@linux.vnet.ibm.com>
Date:   Thu, 15 Jun 2017 06:35:21 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
To:     Will Deacon <will.deacon@....com>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vineet Gupta <vgupta@...opsys.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "David S. Miller" <davem@...emloft.net>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com
Subject: Re: [HELP-NEEDED, PATCH 0/3] Do not loose dirty bit on THP pages



On Wednesday 14 June 2017 10:25 PM, Will Deacon wrote:
> Hi Aneesh,
> 
> On Wed, Jun 14, 2017 at 08:55:26PM +0530, Aneesh Kumar K.V wrote:
>> On Wednesday 14 June 2017 07:21 PM, Kirill A. Shutemov wrote:
>>> Vlastimil noted that pmdp_invalidate() is not atomic and we can loose
>>> dirty and access bits if CPU sets them after pmdp dereference, but
>>> before set_pmd_at().
>>>
>>> The bug doesn't lead to user-visible misbehaviour in current kernel, but
>>> fixing this would be critical for future work on THP: both huge-ext4 and THP
>>> swap out rely on proper dirty tracking.
>>>
>>> Unfortunately, there's no way to address the issue in a generic way. We need to
>>> fix all architectures that support THP one-by-one.
>>>
>>> All architectures that have THP supported have to provide atomic
>>> pmdp_invalidate(). If generic implementation of pmdp_invalidate() is used,
>>> architecture needs to provide atomic pmdp_mknonpresent().
>>>
>>> I've fixed the issue for x86, but I need help with the rest.
>>>
>>> So far THP is supported on 8 architectures. Power and S390 already provides
>>> atomic pmdp_invalidate(). x86 is fixed by this patches, so 5 architectures
>>> left:
>>>
>>>   - arc;
>>>   - arm;
>>>   - arm64;
>>>   - mips;
>>>   - sparc -- it has custom pmdp_invalidate(), but it's racy too;
>>>
>>> Please, help me with them.
>>>
>>> Kirill A. Shutemov (3):
>>>    x86/mm: Provide pmdp_mknotpresent() helper
>>>    mm: Do not loose dirty and access bits in pmdp_invalidate()
>>>    mm, thp: Do not loose dirty bit in __split_huge_pmd_locked()
>>>
>>
>>
>> But in __split_huge_pmd_locked() we collected the dirty bit early. So even
>> if we made pmdp_invalidate() atomic, if we had marked the pmd pte entry
>> dirty after we collected the dirty bit, we still loose it right ?
>>
>>
>> May be we should relook at pmd PTE udpate interface. We really need an
>> interface that can update pmd entries such that we don't clear it in
>> between. IMHO, we can avoid the pmdp_invalidate() completely, if we can
>> switch from a pmd PTE entry to a pointer to PTE page (pgtable_t). We also
>> need this interface to avoid the madvise race fixed by
> 
> There's a good chance I'm not following your suggestion here, but it's
> probably worth me pointing out that swizzling a page table entry from a
> block mapping (e.g. a huge page mapped at the PMD level) to a table entry
> (e.g. a pointer to a page of PTEs) can lead to all sorts of horrible
> problems on ARM, including amalgamation of TLB entries and fatal aborts.
> 
> So we really need to go via an invalid entry, with appropriate TLB
> invalidation before installing the new entry.
> 

I am not suggesting we don't do the invalidate (the need for that is 
documented in __split_huge_pmd_locked(). I am suggesting we need a new 
interface, something like Andrea suggested.

old_pmd = pmdp_establish(pmd_mknotpresent());

instead of pmdp_invalidate(). We can then use this in scenarios where we 
want to update pmd PTE entries, where right now we go through a 
pmdp_clear and set_pmd path. We should really not do that for THP entries.


W.r.t pmdp_invalidate() usage, I was wondering whether we can do that 
early in __split_huge_pmd_locked().

-aneesh